Recent technological advances in microscopic fabrication have made it possible to provide a microscopic nanoscale structure on surfaces of molded articles. Especially, a microscopic pattern, which is referred to as a “moth-eye structure,” is known to be capable of expressing antireflective effects by forming a region of graded refractive indices increasing from the refractive index of air to that of a material. Optical films having a surface with a microscopic pattern possessing a cycle shorter than the wavelength of visible light, namely, having a moth-eye structure, are drawing attention because of their antireflective properties.
In addition to antireflective properties, because of their original properties such as water repellency, referred to as a lotus effect, industrial applications of microscopic nanoscale structures have been intensively studied.
Various technologies are employed to provide a microscopic pattern on surfaces of molded articles. Among them, a method for transferring a microscopic pattern formed on a mold to a surface of a molded article is suitable for industrial production, since such a simplified method requires fewer steps to provide a microscopic pattern. In recent years, a method for using an oxide layer having multiple pores obtained by anodizing an aluminum substrate (anodic porous alumina) has been cited as a simplified method for producing a mold with a large surface area having a microscopic pattern.
To produce anodic porous alumina, an anodizing process is preferred to be conducted in two steps to achieve both a desired pore depth and an ordered array of pores; namely, by a method for forming desired pores on a mold by consecutively conducting steps (I)˜(III) below.
(I): a step for forming an oxide layer by anodizing a surface of an aluminum substrate to obtain an ordered array of pores without paying attention to their depths;
(II): a step for removing a portion of or the entire oxide layer formed in step (I); and
(III): after step (II), the aluminum substrate is anodized again to form pores with a desired depth while maintaining the ordered array.
In the method above, anodic porous alumina having a desired porous pattern is obtained by conducting step (III) which repeats step (I) for forming an oxide layer having multiple pores by anodizing an aluminum substrate and step (II) for enlarging the diameter of the pores. For example, patent publication (1) discloses a method for forming tapered pores in a metal substrate by repeating an anodizing process and an etching process.